1. Field of the Invention
The present invention relates to a photopolymerizing composition and a photopolymerizing recording medium manufactured using the same, and more particularly, to a photopolymerizing composition and a photopolymerizing recording medium that is manufactured using the composition and can be used to manufacture a 3D, ultra-high information storage capacity optical memory.
2. Description of the Related Art
Holograms, which are created by recording an interference pattern of coherent laser light on, for example, a photosensitive material, are multifunctional, so they have various applications in the optical memory, 3D-image display, image processing fields, etc.
In particular, optical memories can be manufactured by forming a deep hologram using a refractive index variation induced due to a variation in the density of a substance composing a thick photopolymerizing recording medium. The hologram can be generated by photopolymerizing a photosensitive recording layer, which contains a photopolymerizable monomer, of the recording medium by exposing the layer to a laser light of proper intensity. The degree of polymerization of the photosensitive layer depends on the intensity of incident active light. The generated image having a set of lines of different intensity is recorded based on a polymer density variation. The generated polymer, having a refraction factor that differs from the refraction factor of an initial monomer, will be characterized by a refraction factor depending on radiation intensity on different areas of the photosensitive layer. These differences are used for recovering an image using light wave phase recovery. A deep hologram can be recorded as a result of photobleaching of a photosensitive component in the recording medium, i.e., penetration of laser light into the recording medium, in a hologram recording process. Deep holograms recorded in the thick layer determine a high angular selectivity of holograms and image recovery. A higher angular selectivity leads to a larger high information storage capacity.
Photopolymerizing media using a monomer-oligomer composition and a photoinitiating system are widely known. A photopolymerizing medium further contains a photobleaching dye and a coinitiator (Carretero L., Blaya S., Mallavia R., et al., Appl. Opt. 1998. Vol. 37, p. 4496). Post-exposure and thermal treatments are required to obtain a phase hologram, thereby making it difficult to use an archive optical memory device.
U.S. Pat. No. 5,230,986 discloses a photocurable composition composed of a free radical-polymerizable compound, an electron-donating coinitiator, and a photochromic benzospiropyran compound of formula (1) below. This benzospiropyran compound undergoes ring opening when exposed to a first actinic radiation or heat, forming a merocyanine compound of formula (II) below. This merocyanine compound generates free radicals when exposed to a second actinic radiation. The free radicals generated from the benzospiropyran compound induce photopolymerization of the polymerizable compound, thereby curing the composition.

In formulae (I) and (II), each of X1 and X3 is independently selected from the group consisting of hydrogen, iodo, nitro, cyano, bromo, chloro, fluoro, and amino, at least one of the two groups X1 and X3 is not hydrogen, and X2 is selected from the group consisting of hydrogen, alkoxy, carboxy, ester, and amino.
Referring to the above reaction scheme, the benzospirane compound undergoes a reversible intramolecular transformation by UV light irradiation or heating and forms a merocyanine isomer. When the colored merocyanine form is irradiated by visible light in the presence of a reducing coinitiator and an olefinically unsaturated compound, polymerization initiating radicals are generated. The colored merocyanine form can act as a visible light photoinitiator. Photopolymerization can be achieved by visible light absorbed by the merocyanine isomer. A photopolymerizing recording medium using such a photopolymerizing polymer is spontaneously desensitized when UV radiation is interrupted or when cooling the photosensitive layer.
Disadvantageously, such a photopolymerizing recording medium requires an additional light source or heat energy when a hologram is recorded by visible laser light, thereby complicating the structure of a 3D imaging apparatus. Constant UV irradiation or heating maintains the concentration of the merocyanine isomer constant, thereby preventing deep light penetration into the recording medium and disabling recording of a deep hologram. As a result, both angular selectivity and information storage capacity decrease. In addition, a thermal distortion of interference patterns occurs as the recording medium is heated, thereby reducing resolution capability. Since the recording medium is in liquid state, a continuous post-exposure process is required after the recording of the hologram, thereby increasing power consumption and the optical memory formation time.
These disadvantages are caused due to an insufficient lifespan of the photoinduced structure of the photochromic compound, which is used as a photosensitizer in the photopolymerizing composition used to form a recording layer of the photopolymerizing recording medium. Moreover, the photopolymerizing composition does not contain a polymer binder or components that facilitate media photopolymerization during recording of a hologram.